Power distribution systems are well known. In a typical arrangement of a marine power distribution and propulsion system shown in FIG. 1 a plurality of ac generators G1 . . . G4 provide ac power to a busbar or switchboard 2 which carries a distribution voltage such as 690 V. Each generator G1 . . . G4 is coupled to a prime mover such as a diesel engine D1 . . . D4.
Electric propulsion motors M1 . . . M4 are connected to the busbar 2 by means of interposing power converters 4. The propulsion motors M1 . . . M4 can be of any suitable type and construction and can optionally be configured to drive a propeller shaft or other propulsion system such as a thruster.
In some arrangements then active front end (AFE) power converters can be used. An AFE power converter typically includes a first active rectifier/inverter 6 having ac terminals connected to the busbar 2 and a second active rectifier/inverter 8 having ac terminals connected to the propulsion motor M1 . . . M4. The dc terminals of the first and second active rectifier/inverters 6, 8 are connected together by a dc link 10. A harmonic filter 12 is normally connected to the ac terminals of the first active rectifier/inverter 6, i.e. on the network-side, to ensure harmonic voltages and currents are eliminated. The AFE power converters might, for example, be implemented as MV3000 converters supplied by GE Energy Power Conversion UK Ltd of Boughton Road, Rugby, United Kingdom.
In normal operation, the first active rectifier/inverter 6 will operate as an active rectifier to supply power to the dc link 10 and the second active rectifier/inverter 8 will operate as an inverter to supply power to the associated propulsion motor M1 . . . M4. However, reverse operation can be possible in certain circumstances such as regenerative braking for the propulsion motors M1 . . . M4.
Each active rectifier/inverter 6, 8 will typically have a suitable topology with semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy.
The busbar 2 can be equipped with protective switchgear with circuit breakers and associated controls. The busbar 2 will typically be divided into a pair of busbar sections 2a, 2b (e.g. port and starboard) that are interconnected by a tie 14. The actual arrangement of the power distribution system will typically depend on redundancy, which is particularly important for marine vessels.
The generators G1 . . . G4 and power converters 4 can be connected to the busbar 2 by circuit breakers 16, 18 and associated controls or other switching means.
A conventional power distribution system can have any suitable number and type of generators and any suitable busbar configuration depending on the power generation and distribution requirements.
In a conventional power distribution system the busbar 2 carries a fixed-frequency distribution voltage, typically 50 or 60 Hz. The frequency of the output voltage of each generator G1 . . . G4 is determined by its shaft speed and must be maintained at the nominal frequency of the power distribution system by the diesel engines D1 . . . D4. For example, a four-pole generator must be maintained at 1500 rpm or 1800 rpm if its output voltage is to have a frequency of 50 or 60 Hz, respectively, to match the particular system frequency.
More generally, the following formula can be used to determine the rotational speed of a prime mover (and hence the rotor of the generator to which it is coupled) for a given system frequency:N=120×F/P  (EQ1)where:    N is the rotational speed of the prime mover in rpm (or shaft speed),    F is the system frequency in Hz, and    P is the number of generator poles.
It will therefore be readily appreciated that any function that controls the rotational speed of the prime mover can also be considered in terms of a frequency control function and vice versa.
With reference to FIGS. 2 and 3, each diesel engine D1 . . . D4 is typically provided with an electronic speed controller 20 (often called a governor) to regulate its rotational speed. The speed controller 20 varies the output torque of the diesel engine automatically in response to detected changes in the shaft speed by controlling the fuel delivery system of the diesel engine, e.g. by opening and closing the fuel rack actuator 22 or by varying the air flow/injector duty-cycle on common rail diesel engines.
As the load demand on the busbar 2 changes then the mechanical torque requirement on the generators G1 . . . G4 will change accordingly. The shaft speed of each generator G1 . . . G4 is monitored and the speed signal is compared against a speed reference or setpoint as described in more detail below. Any deviation from the speed reference is detected by the associated speed controller 20 and the fuel delivery system 22 can be controlled accordingly. For example, if the shaft speed of the first generator G1 decreases then more fuel can be delivered so that the output torque of the associated diesel engine D1 is increased to meet the required load torque or vice versa to restore the shaft speed (and hence the frequency of the generator output voltage) to the nominal. Each speed controller 20 can therefore be considered as a closed-loop regulator.
If just a single generator is connected to a large supply network such as a utility grid (e.g. a so-called ‘infinite network’) and the system frequency falls due to high load demand then the speed controller in its basic form would simply increase its speed reference in an attempt to restore the system frequency to nominal. In practice, the system frequency is not influenced by the generator and the speed controller would simply go to its maximum setting and the generator would most likely trip on overload. If the system frequency increases due to reduced load demand then the fuel delivery system would most likely be reduced to zero.
The same situation occurs when two or more generators are connected to operate in parallel with each other. In this case the system characteristic will be somewhere between an infinite network and a single-connected generator.
To enable stable parallel operation of the generators G1 . . . G4, each speed controller 20 includes a speed droop control function which has the effect of reducing the speed reference as the active power (kW) or load increases or vice versa. By operating in this manner it is possible to achieve stable load sharing between parallel-connected generators or with an infinite network. The active power produced by each generator G1 . . . G4 is monitored by the associated speed controller 20 by means of a transducer in the busbar 2 which converts the load signal to an active power feedback signal kWf. The active power feedback signal kWf is converted to a speed signal by a controller K which is then subtracted from an initial speed error signal δN1 (i.e. the difference between a speed feedback signal Nf that is derived directly from the shaft speed and the speed reference or setpoint Ns that is applied by the power management controller 24—see below) in order to modify the shaft speed of the associated diesel engines D1 . . . D4 for the given active power. In particular, the resulting second speed error signal δN2 is used by the speed controller 20 as a speed reference to control the fuel delivery system 22 of the diesel engine.
A typical value of droop is 3-5% which means that the shaft speed will fall between 3-5% for an increase in generator load from zero to rated load. The amount of droop applied is set at the speed controller 20 by means of an internal gain in the controller K.
FIG. 4 shows how speed droop control can be applied to a single generator where an increase in the speed reference causes a corresponding increase in the speed of the generator and vice versa and where generator speed falls as load increases or vice versa. Note that as mentioned briefly above: Droop (%)=100×(No load speed−Full load speed)/No load speed. FIG. 5 shows how two parallel-connected generators A and B with identical speed references and speed droop control will share the load equally (PA=PB=½AB). Both generators are locked in synchronisation and their speeds are therefore identical. The common speed N (and hence the system frequency) is at the point where the two droop lines intersect. Finally, FIG. 6 shows the effect of changing the speed reference for one of the parallel-connected generators. An increase in the speed reference for generator B will cause its speed to increase from N to N′ with generator B taking a greater share of the load (P′B) and generator A taking a smaller share of the load (AP′). The system frequency can be restored back to nominal by simultaneous reduction in the speed references for generators A and B.
A power management controller (or power management system (PMS)) 24 can be used to coordinate the operation of the diesel engines D1 . . . D4, and in particular the adjustment of the speed reference Ns to ensure that the generators G1 . . . G4 take equal amounts of active power in proportion to their output (or ‘nameplate’) rating.
Ensuring that the generators G1 . . . G4 are loaded equally allows the power management controller 24 to apply a start/stop control function where generators are connected to, or disconnected from, the busbar 2 to meet changing load demands. For example, the power management controller 24 can initiate the starting of one or more additional generators and connect them to the busbar (i.e. bring them on-line) if the loading on any one of the connected generators exceeds a pre-determined threshold, e.g. 90% of its output rating. The power management controller 24 can also initiate the disconnection and shutdown of one or more generators if the loading on any of the connected generators falls below a pre-determined threshold, e.g. 30% of its output rating. In the case of a marine power distribution and propulsion system then the thresholds can be determined with reference to the overall power requirements of the marine vessel (e.g. its intended purpose or duty), but any power distribution system should ensure that a sufficient margin of spare capacity—otherwise known as the ‘spinning reserve’—is maintained. This allows the power distribution system to meet any sudden increases in load demand, and to prevent the unnecessary starting and stopping of generators in response to minor changes in load demand which might occur during normal operation.
The power management controller 24 can include generator-specific control functions (i.e. those control functions that are applied to each individual generator or its associated prime mover) and common control functions that are applied to all of the generators. In FIGS. 2 and 3 the generator-specific control functions are grouped together in a generator-specific controller 26 while the common control functions are grouped together in a common controller 28.
It will be readily appreciated that the common control functions include the start/stop control function described above, together with other functions such as the automatic removal or load-shedding of non-essential loads under overload conditions etc. However, for clarity only a load sharing control function and an operator control function are shown in FIG. 3 for a single generator G1 and its associated diesel engine D1.
The load sharing control function uses information data indicative of the number of generators that are on-line, their kW rating, their actual loading—to determine the total system load, together with other operational parameters of the power distribution system such as the number of operational busbar sections and the open/closed status of the circuit breakers (‘CB STATUS’) or protective switchgear. This information data is provided to a load share function block 30 which calculates an active power (kW) reference or setpoint kWs and a reactive power (kVAr) reference or setpoint (not shown in FIGS. 2 and 3) based on the power distribution system configuration. The active power and reactive power references are distributed for use in the generator-specific controllers 26 of the power management controller 24.
The operator control function uses data from a human machine interface (HMI) which can be in the form of a workstation 32 and which allows an operator to make changes to the operational parameters of the power distribution system such as manually starting or stopping, synchronising or disconnecting generators, modifying the voltage and frequency references or setpoints etc. The operator control function shown in FIG. 3 is a frequency control function which allows the operator to modify the system frequency. A frequency reference Fs is normally set to 50 or 60 Hz which equates to the nominal speed of the associated generator—see equation EQ1 above—and is only changed during commissioning of the power distribution system or in extreme cases where a change in system frequency is needed due to a reduction in engine performance, e.g. a reduction in transient load performance. A frequency feedback signal Ff is derived at the generator output and is filtered by a simple first-order filter 34 to remove noise and averaged by a moving average filter 36. The frequency reference Fs set by the operator is compared against the averaged frequency feedback signal Ffav and the resulting frequency error signal δf is applied to a deadband function block 38, typically set at about 0.2 Hz. The output of the deadband function block 38 is a frequency control signal δf1 that is distributed for use in the generator-specific controller 26 of the power management controller 24.
The generator-specific control functions will now be described. Although FIG. 3 only shows the generator-specific controller for the first generator G1 it will be readily appreciated that corresponding control functions will be provided for each of the remaining generators G2 . . . G4 and will use feedback signals derived at the output of the respective generator as shown in FIG. 2.
An active power feedback signal kWf is derived at the output of the respective generator and is filtered by a simple first-order filter 40 to remove noise and averaged by a moving average filter 42. (It will be readily appreciated that averaging the active power and frequency feedback signals kWf and Ff is important because it allows transients to be ignored.)
The averaged active power feedback signal kWfav is compared against the active power reference kWs that is provided by the function block 30 and the resulting active power error signal δkW is applied to a function block 44 which converts the error signal to a second frequency error signal δf2 with reference to the droop characteristic (ekW) of the speed controller 20. The first and second frequency error signals δf1 and δf2 are summed to give a combined frequency error signal δfc which is multiplied in a function block 46 by the droop characteristic (ekW) of the speed controller 20 to convert it back to an active power error signal. It will be noted that the first frequency error signal δf1 is also distributed to the other generator-specific controllers 26 to make sure that the desired system frequency is achieved without affecting the kW load balance between the parallel-connected generators G1 . . . G4.
The active power error signal from the function block 46 is then applied to a deadband function block 48 before being converted to speed raise or lower command signals by function block 50. These command signals are provided to the speed controller 20 of the respective diesel engine D1 . . . D4 which incorporates an internal ramp function R shown in FIG. 3 that converts the digital pulses to an analogue speed reference or setpoint Ns. It will be noted that the duration of the speed raise or lower command signals from each generator-specific controller 26 is matched to the acceleration or deceleration rate of the speed controller 20, which is itself matched to the actual acceleration or deceleration rate of the respective diesel engine. This ensures accurate speed and power control of the diesel engines D1 . . . D4 for the purposes of maintaining the correct active power load sharing of individual generators, and frequency control across the common busbar or switchboard thereby negating the speed droop effect of the speed controllers.
As shown in FIG. 7 the efficiency of a fixed-speed diesel engine varies with load. More particularly, the higher the load the lower the specific fuel oil consumption (SFOC) of the diesel engine (or the corresponding measure for any other type of prime mover). This highlights a particular weakness in conventional power management controllers where the connection of additional generators to meet increasing load demands reduces the operational efficiency of the on-line generators, thus reducing the overall efficiency of the power distribution system.
FIG. 7 also shows that for a particular operating load there is an optimum speed setting to minimise fuel oil consumption. An extreme example would be running a diesel engine at 100% rated speed at 20% load. In this instance the SFOC would be in the region of 270 g/kWh. Running the diesel engine at 50% rated speed for the same load would reduce the SFOC to about 230 g/kWh—resulting in a 15% reduction in fuel oil consumption. A reduction in SFOC also corresponds to a reduction in environmentally harmful exhaust gases such as nitrogen oxides (NOx), carbon dioxide (CO2) and other pollutants related to the combustion process. However, such a speed reduction is not possible during normal operation of a conventional power distribution system which operates at a fixed nominal frequency.
It can therefore be seen that there is a need for a more efficient power distribution system.